1. Field of the Invention
The present invention relates to a substrate-cleaning method used for cleaning of a substrate which has a metal material and a semiconductor material both exposed at the surface, as well as to a substrate-cleaning solution used in the substrate-cleaning method.
2. Description of the Related Art
In the process for formation of damascene interconnection and leveling of via hole, chemical mechanical polishing (CMP) has been in wide use in recent years.
CMP is a technique used for leveling of film, wherein a wafer having an inter-layer insulating film and a metal material film formed thereon is pressed against a buff (a cloth) and rotated while feeding a slurry (a mixture of abrasive particles and a chemical reagent) and thereby the inter-layer insulating film and the metal material film are polished and leveled by the combination of a chemical action and a mechanical action.
When a metal interconnection or the like is formed on a substrate by CMP, a large amount of impurities remain on the silicon oxide film and the metal region after the CMP, and this becomes a problem. For example, in CMP of tungsten (W), there is used, for good polishing rate and processing accuracy, a solution which is a mixture of (1) a mixed solution of aqueous hydrogen peroxide and an oxidizing agent (Fe) and (2) an abrasive (silica particles or alumina particles); after the blanket W has been removed by CMP, a large amount of particles (silica particles, alumina particles, etc.) and metal impurities (Fe contaminant, etc.) remain on the exposed surface of silicon oxide film. Ordinarily, the number of remaining silica particles or alumina particles is 30000 per wafer or more and the atoms of remaining Fe is 1012 per cm2 or more.
Remaining of impurities on wafer gives rise to cross-contamination in the subsequent steps (e.g. the step for formation of inter-layer insulating film, the step for formation of interconnection, etc.), and this becomes a problem. For example, the inside of a CVD apparatus (where an inter-layer insulating film is formed) is contaminated, or disconnection of aluminum interconnection is invited. Therefore, it is strongly desired to develop a method for efficiently removing, by cleaning, the impurities remaining on a wafer after CMP.
Thus, when a substrate is subjected to CMP, it is necessary to efficiently remove, by cleaning, the impurities adhering to the substrate after the CMP. This necessity exists in cleaning of all substrates having a metal interconnection, a via hole, etc. formed thereon. For such substrate cleaning, so-called RCA cleaning is in wide use. The RCA cleaning comprises (1) a step of removing particles by cleaning with ammonia-aqueous hydrogen peroxide and (2) a step of removing metal impurities by the use of hydrochloric acid-aqueous hydrogen peroxide, DHF (diluted hydrofluoric acid)), citric acid or the like. Thus, in the RCA cleaning, the cleaning is conducted in two steps and thereby the particles and metal impurities remaining on a substrate can be efficiently removed without impairing the substrate.
Further, there is disclosed, in JP-A-10-72594, a method for cleaning of a substrate having a metal interconnection or the like formed thereon, by using a cleaning solution containing (a) an organic acid such as citric acid, oxalic acid or the like and (b) a complexing agent such as aminopolycarboxylic acid or the like. In the method, particles and metal impurities are said to be removed without causing the corrosion of metal interconnection or impairing the surface flatness of semiconductor substrate.
Meanwhile, in JP-A-8-187475 is disclosed a method for cleaning of a silicon substrate by using a cleaning solution containing a surfactant (this method is different from the method for cleaning of a substrate having a metal interconnection formed thereon).
The above-mentioned conventional cleaning techniques, however, have had the following problems.
In the conventional RCA cleaning, metal impurities are removed by the second cleaning (acid cleaning). When the acid used in the acid cleaning is hydrochloric acid-aqueous hydrogen peroxide or DHF (diluted hydrofluoric acid), the acid has too strong a dissolving power for metals and corrodes the metal interconnection or the like exposed at the substrate surface. Meanwhile, when the acid used in the acid cleaning is citric acid, the acid hardly causes the above problem, but is insufficient in cleaning power and, in some cases, has been unable to completely remove the metal impurities present on the substrate. In the case of citric acid, there has also been a problem in that a large amount of citric acid must be used for securing a sufficient cleaning power, making high the TOC (total organic carbon) of the used cleaning solution.
In the conventional RCA cleaning, there has further been a problem in that the first cleaning (alkali cleaning) invites the adhesion of metal impurities to substrate surface, making difficult the removal of the adhered metal impurities. This problem is explained below in the case of a silicon substrate having a copper interconnection formed thereon. When the first cleaning (alkali cleaning) has been conducted by brush-cleaning with ammonia water (concentration: 0.1 to 2% by weight), the Cu interconnection exposed on the substrate surface is etched and pits may be formed in the Cu interconnection film. This happens because the ammonia water forms an ammonia complex with Cu and thereby the dissolution of Cu takes place. Since the Cu interconnection film is in the form of polycrystal, the etching rate is larger in the grain boundaries and pits are formed. When metal impurities adhere onto the pits, the metal impurities are very difficult to remove by the second step (acid cleaning). In filling W or Cu in the groove or plug of substrate, pores may be formed inside the W or Cu filled in the groove or plug. Once impurities such as particles and metals enter the pores during CMP, they are difficult to remove by cleaning.
When a metal wiring or the like is formed by CMP, the removal of the metal impurities adhering to the metal region of substrate after CMP is difficult also for other reasons. This matter is explained below.
When a metal interconnection or the like is formed by CMP, so-called dishing takes place ordinarily. The dishing is a phenomenon that a dent is generated at the center of the surface of a filled conductive film. FIG. 9 is a drawing showing a state in which a dished part 23 has been generated in the formation of a Cu interconnection 1 by CMP. An inter-layer insulating film 3 is formed on a silicon substrate 4; in a given portion of the inter-layer insulating film 3 is filled a Cu interconnection 1 via a barrier film 2 (a TiN layer). In the step of formation of this Cu interconnection 1, the unnecessary parts of the Cu interconnection 1 and the barrier film 2 (a TiN layer) are removed by CMP; at that time, the center of the surface of the Cu interconnection 1 is polished and dented and, as a result, a dished part 23 is generated. The dished part 23 appears because the polishing rate of Cu is far greater than that of TiN. In other words, when TiN on the inter-layer insulating film 3 is removed, the polishing of Cu layer (this layer has a larger polishing rate than TiN) proceeds excessively and thereby dishing takes place. In order to completely remove the unnecessary part of TiN, slight overetching is necessary and, therefore, dishing appears more or less in many cases. Dishing is striking when the length of conductive layer (i.e. the length of interconnection) is large.
Generation of dished part invites adhesion of impurities thereto and the removal of the adhered impurities becomes difficult.
Generation of dished part may incur formation of a different level part at the boundary between the barrier and the filled conductive film (see the enlarged view of FIG. 9). Impurities (e.g. particles and metals) adhere easily to this different level part and the adhered impurities are difficult to remove by later cleaning.
Thus, when CMP is applied to a substrate having a metal interconnection layer or a via hole, a dished part is generated, making very difficult the removal of particles and metal impurities. That is, the impurities adhering to the dished part of metal film or the different level part between barrier film and metal film are not removed by conventional acid cleaning; even when physical cleaning (e.g. brush cleaning using a rapidly rotating brush) is used in combination with the acid cleaning, the shear force of the brush does not reach the dished part and no sufficient removal is attained.
Meanwhile, in the cleaning method described in 10 JP-A-10-72594, a complexing agent such as aminopolycarboxylic acid or the like is used to achieve improved removal of metal impurities. In this cleaning method, however, no sufficient cleaning is obtained for the ever increasing requirements for substrate cleaning, and removal of, in particular, the metal impurities adhering to the dished part after CMP is very difficult.
As described above, when a substrate having a metal interconnection or the like is subjected to CMP and then cleaned, there is a problem which does not exist in cleaning of a substrate having no metal interconnection or the like, because the former substrate has an exposed metal region and a dished part is generated in the metal region.
Further, when a substrate having a metal interconnection or the like is subjected to CMP and then cleaned by, for example, the above-mentioned RCA cleaning, ordinarily particles are removed by the first step (alkali cleaning) and metal impurities are removed by the second step (acid cleaning); in this case, there is another problem which does not exist in cleaning of a substrate having no metal interconnection or the like, because pits are generated in the metal region of the former substrate.
The conventional RCA cleaning and the cleaning described in JP-A-10-72594 provide no sufficient solution to the above-mentioned problems. The cleaning method described in JP-A-8-187475, which is for cleaning of a substrate having no exposed metal region, provides no solution for the above problems which exist in cleaning of a substrate having an exposed metal region.
The present invention aims at providing a substrate-cleaning method which efficiently removes the particles and metal impurities present on a substrate having a metal material and a semiconductor material both exposed at the surface, particularly the metal impurities adhering to the surface of the metal region of the substrate and which can achieve higher cleanliness of substrate than before.
According to the present invention there is provided a substrate-cleaning method used for cleaning of a substrate which has a metal material and a semiconductor material both exposed at the surface and which has been subjected to a chemical mechanical polishing treatment with a polishing solution, said method comprising the steps of:
(A) a first step of cleaning said substrate with a first cleaning solution containing ammonia water or an aqueous catholyte generated in electrolysis, and
(B) a second step of cleaning the substrate after the first step (A) with a second cleaning solution containing (a) a first complexing agent capable of easily forming a complex with the oxide of said metal material or the oxide of the metal contained in said polishing solution, and (b) an anionic or cationic first surfactant.
According to the present invention there is further provided a substrate-cleaning solution used for cleaning of a substrate which has a metal material and a semiconductor material both exposed on the surface, which cleaning solution contains (a) a first complexing agent capable of easily forming a complex with-the oxide of said metal material and (b) an anionic or cationic surfactant.
According to the present invention there is furthermore provided a substrate-cleaning solution used for cleaning of a substrate which has a metal material and a semiconductor material exposed on the surface and which has been subjected to a chemical mechanical polishing treatment with a polishing solution, which cleaning solution contains (a) a first complexing agent capable of easily forming a complex with the oxide of said metal material, (b) an anionic or cationic surfactant, and (c) a second complexing agent capable of easily forming a complex with the metal impurities remaining on the surface of the substrate subjected to the chemical mechanical polishing treatment.
The second cleaning solution used in the substrate-cleaning method of the present invention and the substrate-cleaning solution of the present invention contain an anionic or cationic surfactant. This surfactant functions so as to (1) make hydrophilic the surface of the substrate to be cleaned and make easy the detachment of the metal impurities present on the surface, from the surface and (2) prevent the readhesion of the detached metal impurities to the substrate surface. The second cleaning solution used in the substrate-cleaning method of the present invention and the substrate-cleaning solution of the present invention further contain a first complexing agent capable of easily forming a complex with the metal material exposed at the substrate surface. This complexing agent functions so as to make easy the detachment of the metal impurities present on the substrate surface, from the surface and acts effectively particularly on the metal impurities adhering to the metal region of the substrate. The complexing agent adheres easily particularly to metals. The complexing agent can efficiently remove, in particular, the metal impurities adhering to the pit generated in the metal region as a result of the alkali treatment [the first step (A)] and the metal impurities adhering to the dished part of the metal region formed by CMP. Further, the complexing agent can make easy the removal of the metal impurities present in the pore which may be formed inside the metal region filled in the groove or hole of substrate and can effectively prevent the readhesion of the impurities. The surfactant used in the present invention makes adequately hydrophilic the surfaces of the pit, the dished part and the pore, and its synergism with the complexing agent promotes the detachment of metal impurities. Further, the surfactant covers the surfaces of the metal impurities detached and thereby can prevent the readhesion of the impurities to the metal region.
The surfactant can exhibit a high cleaning effect when used in combination with the complexing agent. When the surfactant is used singly, no sufficient cleaning effect is obtainable and the removal of the metal impurities adhering to the substrate metal region is difficult.
As described above in 2. Description of the Related Art, the technique of using a surfactant in cleaning a silicon substrate is already known. In contrast, the present invention has a feature in that a combination of a surfactant and a complexing agent is used in cleaning of a substrate having a metal material exposed at the surface. This combined use of a surfactant and a complexing agent can exhibit an excellent effect for removal of the metal impurities adhering to the metal region of substrate, which effect has been unobtainable with the single use of a surfactant or a complexing agent. The combination of a surfactant and a complexing agent exhibits a noticeable effect for removal of, in particular, the metal impurities adhering to the pit generated by the first cleaning using an alkali and the metal impurities adhering to the dished part generated in CMP.